Automatic exposure control (AEC) is used in X-ray imaging equipment to control the exposure per image. The goal is to maintain image quality while minimizing patient exposure. The AEC develops a signal proportional to the X-ray flux into the image receptor. This signal is used to regulate the total exposure for each image either by terminating the exposure or by adjusting the X-ray flux rate. In this usage, the X-ray technique (kVp, spectral filter, focal spot, etc) is prescribed by the operator. Often these are preset and selected based on patient size and anatomy to be imaged. Thus, the role of the AEC is to regulate the correct total exposure.
Generally, an incident radiation dose must be adjusted for each object or every imaging. Conventionally, an AEC control sensor is arranged independently of the imaging radiation detector. Pluralities of thin AEC sensors that attenuate radiation by about 5% are separately arranged in front of the imaging radiation detector. Incidence of radiation is stopped based on the outputs from the AEC sensors, thereby obtaining an appropriate radiation dose for imaging. As an AEC sensor, a sensor which directly extracts radiation as charges by using an ion chamber, or a sensor which converts radiation into visible light through a phosphor, extracts the visible light through an optical fiber, and causes a photomultiplier to convert the visible light into charges is used. This analog output is routed over a long cable to the X-ray control where the signal is compared to a reference voltage representing the required exposure. The typical interface to the ion chamber includes an ion chamber selection, an exposure start signal, and a power supply as inputs and an analog integrated brightness as the output.
The spectral sensitivity of these devices is neither constant over the required kVp range nor can it exactly match the characteristics of the recording medium. The recording medium may be a film screen cassette, CR cassette or a flat panel detector.
The temporal response of the ion chamber design does not account for limitations in the exposure control system. Very long exposures for film require system level AEC compensation to increase the required exposure to compensate for reciprocity loss, also called the Swartzchild effect. Very short exposures require an anticipation function to reduce the required exposure to compensate for the delays in the ion chamber itself as well as rise and fall time limitations in the X-ray generator.
The spectral and temporal response characteristics require calibration at a system level to provide acceptable system performance. In some systems the required compensations are partially incorporated in the design of the system level AEC. In some cases the AEC calibration is one of the longer tasks in the system calibration process.
Yet another drawback with conventional automatic exposure control is when ion chamber characteristics are embedded in the system level AEC design, then changing ion chamber designs or suppliers requires a new software release to incorporate any differences in performance.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an integrated automatic exposure control (AEC) subsystem. There is also a need for improved signal to noise, accuracy and small signal sensitivity in an automatic exposure control (AEC).